The present invention relates generally to an illumination optical system for illuminating a target surface with light from a light source, an exposure apparatus, and a device fabrication method, and more particularly to an illumination optical system for illuminating a reticle (or a mask) which forms a pattern with short wavelength light. The present invention is suitable for a projection exposure apparatus used in a photolithography process for fabricating semiconductor devices, liquid crystal display devices, image pick-up devices (CCD, and the like), thin-film magnetic heads, and the like.
An exposure apparatus is used in a photolithography process in a semiconductor device fabrication process. The photolithography process is a process of projection transcribing a circuit pattern of the semiconductor device on a substrate (silicon substrate, and the like) that becomes the semiconductor device.
Minute semiconductor devices have been increasingly demanded. The minimum line width of a line and space demanded is 0.15 μm or less, and reaching 0.10 μm. The improvement of a resolution of the projection optical system used for the photolithography process is necessary to create minute semiconductors.
In general, a line width R that can be resolved in the photolithography process can be determined by using an exposure light-source wavelength λ, the numerical aperture NA of the exposure apparatus, and proportionality factor k1 as in the following equation:
                    R        =                              k            1                    ×                      λ            NA                                              (        1        )            
Therefore, if the wavelength λ is shortened, the line width R that can be resolved decreases in proportion to wavelength. Moreover, if the numerical aperture NA is increased, the line width R that can be resolved decreases in inverse proportion to numerical aperture. Recently, producing exposure light of short wavelength and making to the high NA projection optical system were advanced. The exposure light source of the current mainstream projection exposure apparatus is KrF excimer laser with a wavelength of 248 nm. The projection exposure apparatus that is developed using ArF excimer laser (with a wavelength of 193 nm) and F2 laser (with a wavelength of 157 nm). Moreover, the current mainstream NA is about 0.80, but the projection exposure apparatus that is developed has NA of 0.90.
However, a glass material with high transmittance is limited to the short wavelength of the light source, and a glass material that can be used by the wavelength of 157 nm is only a calcium fluoride (CaF2) at present. Thereby, it is examined to use a catadioptric system that combines a dioptric system with a catoptric system as the projection optical system to correct a chromatic aberration with a single glass material.
An optical system that uses a 45° mirror is proposed as an example of the catadioptric system as shown in FIG. 5 (see, for example, Japanese translation of PCT international Application Publication No. 7-111512). The reflectivity of the 45° mirror is different in S-polarized light (a light beam whose direction of electric field vector is vertical to a normal of a reflection surface and traveling direction of the light beam) and P-polarized light (an orthogonal light beam to the s-polarized light). Therefore, the transmittance between the reticle and the wafer is different between the light beam that becomes p-polarized light and the light beam that becomes s-polarized light for the mirror in the catadioptric system.
On the other hand, in the projection exposure apparatus, a contrast of an interference fringe that the line and space of the reticle forms a photoresist on the wafer is known as follows: S-polarized light has higher contrast than P-polarized light for a diffraction light of the line and space.
Therefore, in the projection exposure apparatus that uses the projection optical system of the catadioptric system that has the 45° mirror shown in FIG. 5, if light intensity equal to light intensity of S-polarized light and P-polarized light illuminates the reticle, the contrast changes depending on the pattern direction.
A situation that applies a reticle to form a pattern repeated in y direction to the optical system shown in FIG. 5, is shown in FIG. 6. A situation that applies a reticle to form a pattern repeated in x direction to the optical system shown in FIG. 5 is shown in FIGS. 7A and 7B. A diffraction light of the repeatedly pattern in y direction shown in FIG. 6 is reflected to the x direction, and s-polarized light component of the diffraction light becomes s-polarized light component for the mirror. A diffraction light of the repeated pattern in x direction shown in FIG. 7 is reflected to the y direction, and s-polarized light component of the diffraction light becomes p-polarized light component for the mirror.
The reflectivity of the mirror is different in S-polarized light component and P-polarized light component So S-polarized light component of the diffraction light diffracted by each of the repeated patterns shown in FIG. 6 and the repeated patterns shown in FIG. 7 becomes S-polarized light and P-polarized light respectively for the mirror, and a transmittance that transmits the projection optical system is different. A transmittance that is composed of S-polarized light component and P-polarized light component does not depend on the pattern and is constant, and a light intensity ratio of S-polarized light component and P-polarized light component of the diffraction light that reaches the wafer is different depending on the pattern direction. As mentioned-above, the contrast is different in S-polarized light component and P-polarized light component. If the same light intensity as light intensity of S-polarized light and P-polarized light illuminates the reticle, the contrast difference, that depends upon the pattern direction, is caused in the projection exposure apparatus that uses the projection optical system of the catadioptric system.
Therefore, if the same light intensity as a light intensity of S-polarized light and P-polarized light illuminates the reticle, the contrast is different depending on the pattern direction, and an error occurs (called “HV difference”) that changes depending on the pattern direction. Thereby, the semiconductor device fabrication yield decreases by using the projection exposure apparatus.
Then, a method of illuminating the reticle surface by partial polarized light, and equating compared with the light intensity ratio of S-polarized light and P-polarized light in the wafer surface is proposed (see, for example, Japanese Patent No. 2000-3852).
When forming a predetermined polarization ratio in the illumination optical system, the polarization state changes by a birefringence generated by a birefringence of the glass material of the illumination optical system until the reticle surface and a stress that given the lens when lens is fixed until the stress is reduced to produce the predetermined ratio.
FIGS. 8A and 8B shows a calculation result that the light of S-polarized light at the pupil position of the illumination optical system change into the light intensity distribution (of S-polarized light and P-polarized light) at the reticle surface. This illumination optical system has an optical system that adjusts the polarization ratio at the pupil position, is used for the light source of light with a wavelength of 157 nm, and a calcium fluoride that has birefringence amount of 2 nm/cm on m+2σ (m: average, σ: variance) is used as a lens of the illumination optical system.
A thickness of the glass material from the pupil position of the illumination optical system to the reticle is about 500 mm, and a number of lens is 15. The right side drawing is a pupil distribution of P-polarized light at a predetermined point on the reticle surface. The S-polarized light might change into P-polarized light by the birefringence of the lens even in case of incidence. For example, when the birefringence of the calcium fluoride is 2 nm/cm, maximum 25% P-polarized light is generated at the reticle surface.
FIG. 9 shows a relationship between a birefringence amount of the glass material used for the illumination optical system lens and the maximum P-polarized light intensity in a pupil of the reticle surface made by light of S-polarized light at pupil position of the illumination system. Plural points in the same birefringence amount is one that a built-in angle of lens to a lens barrel was changed, and the maximum P-polarized light intensity in the pupil in the reticle surface changes by combining a phase advance axis. The ratio of S-polarized light changing into P-polarized light increases as the birefringence amount increases as shown on graph on FIG. 9. When assuming that the birefringence of 5 nm/cm is allowed, the light of 90% of S-polarized light at the pupil position of the illumination optical system might change into the P-polarized light.